The present invention relates to a method for producing an integral, asymmetric membrane for the separation of gases from one another, wherein a membrane former, a solvent and a swelling agent are mixed together, spread out over an area to form a thin film, and brought into contact with a precipitation agent. It also relates to the membranes themselves, and to the various shapes in which the membranes may be embodied.
Separation of gases by means of membranes, in principle, is a very promising procedure. Compared to classical gas separation processes, such as cryogenic distillation and sorption processes, gas separation by means of membranes can be effected with noticeably less energy consumption and can take place isothermally without phase changes.
In this separation process, the membrane characteristics constitute a significant cost factor. First, the membrane must have suitable selectivity. If one wishes to separate gas A (e.g., helium or hydrogen) from gas B (e.g., nitrogen, methane or carbon monoxide), the membrane should have a selectivity .alpha. of at least 20. A selectivity of .alpha.A/B=20 means that with identical partial pressures, 20 times more gas A per unit time diffuses through the membrane than gas B.
Second, it is very important that the preferred component diffuse through the membrane at a sufficient flow rate. The greater the gas flow per unit area and unit time, the more economical the separation process will be. Since flow through a membrane is generally inversely proportional to its thickness, it is desirable to keep the thickness of a membrane as small as possible. In connection with the manufacture of very thin films, the limit of feasibility is soon reached, since thin films of a thickness of 1 micron or less are difficult to handle and can, therefore, rarely be manufactured without flaws. Moreover, the membrane cannot have a porous surface, as even micropores in an order of magnitude of 10.sup.-8 have devastating effects on the selectivity of a gas separating membrane.
One possibility for overcoming these difficulties is offered by the manufacture of so-called integral, asymmetric membranes. Such membranes are composed of a porous, voluminous substructure and a very thin (0.1 to 1 micron) skin constituting the actual separating membrane. Skin and substructure are made of the same material, cellulose acetate, and are produced practically in one processing phase.
The discovery of these membranes made desalination of sea water by reverse osmosis economically feasible. After this discovery, there was no lack of experimentation to produce integral, asymmetric membranes for gas separation purposes as well. As in the production of very thin films, it was discovered that it was usually impossible to produce the membrane skin so that it was free of micropores. At the present time, the only integral, asymmetric membrane free of pores that has gained some economic significance for gas separation purposes is a dried cellulose acetate membrane originally developed for sea water desalination (made by Separex, USA). The drying process for the initially water wet membrane involves a relatively complicated solvent exchange. If the wet cellulose acetate membranes are dried directly in air, the porous substructure collapses and a membrane that is almost gas impermeable results.
A further drawback of these finished membranes is their extraordinary water sensitivity. They are irreversibly destroyed by liquid water. Moreover, the membranes cannot be used at elevated temperatures (T&gt;70.degree. C.) since changes in the material considerably reduce selectivity. For cellulose acetate, the maximum attainable He/N.sub.2 selectivity lies at about 85 (average about 60).